At a transmitting station in such a QPSK system, a number n=2P of signal levels are translated into relative phase shifts of two carrier waves of like frequency, nominally in quadrature with each other, whereupon these carriers are combined into an outgoing wave. With p=3, for example, each carrier may be selectively subjected to four different phase delays to establish eight distinct signal levels or symbols. With coherent demodulation, i.e. when the carrier frequency is made available at a receiving station (as by a separately transmitted pilot tone), the two carriers can be readily separated as described, for example, in commonly owned U.S. Pat. Nos. 4,029,903 and 4,076,956. Distortions experienced during transmission are eliminated with the aid of a decision circuit which quantizes the detected phase difference between the two carriers to reconstitute the signal levels represented thereby.
When transmission of the carrier frequency to a receiving station is impractical, the incoming signal at that station must be split into in-phase and quadrature components by non-coherent demodulation, e.g. as described in U.S. Pat. No. 3,935,535; to compensate for intersymbol interference, these components are digitized and fed to a phase comparator by way of a preferably adaptive equalizer advantageously comprising transversal filters. The filtering coefficients, however, must be continuously updated to minimize unavoidable decision errors.